Improving efficiency of semitransparent organic solar cells by constructing semitransparent microcavity

Semitransparent organic solar cells have become attractive recently because of their photon harvesting in the near-infrared and ultraviolet range and passing in the visible light region. Semitransparent organic solar cells with Glass/MoO3/Ag/MoO3/PBDB-T:ITIC/TiO2/Ag/PML/1DPCs structure have been studied in this work and the effects microcavity with 1-dimensional photonic crystals (1DPCs) on the solar cell performance such as the power conversion efficiency, the average visible transmittance, Light utilization efficiency (LUE), the color coordinates in the CIE color space, and CIE LAB are investigated. The analytical calculation including the density of exactions and their displacement is used to model the devices. The model shows that the presence of microcavity can improve the power conversion efficiency by about %17 in comparison with the absence of microcavity. Although the transmission is decreasing slightly, microcavity does not change the color coordinates much. The device can transmit high-quality light with a near-white sensation to the human eye.

Organic solar cells (OSCs) have attracted much attention due to their advantages such as low cost, easy fabrication, flexibility, and recently their potential applications for Semi-transparent Solar Cells (ST-SCs), although their stability is challenging [1][2][3][4][5][6][7] . ST-SCs, combining the benefits of light-to-electricity conversion and light transparency, have emerged as one of the most prominent energy harvesting technologies. These technologies can be used for Agri-voltaic (as the roof of greenhouses), and windows for buildings. The performance of ST-SCs depending on their applications is generally determines by their capability to convert the incident light into electricity while allowing transmitted light through the device.
ST-SCs require not only a transparent active layer but also both electrical contacts, electron transport layer (ETL), and hole transport layer (HTL) to be transparent in a wide spectral range (from IR to UV) along with an efficient collection of photo-generated charge carriers. For front electrodes, transparent conductive oxide 8 , thin metal film [9][10][11] , the conductive polymer 12,13 , graphene 14,15 , and nanotube films 16 have been previously used and also for back electrodes mostly organic active bulk heterojunction layers are used 17 . However, using a transparent active layer and electrodes decreases the device efficiency 9 . To overcome this problem, a method is coating one-dimensional photonic crystals (1DPCs) on the top electrode of ST-SC [17][18][19][20][21][22] . Although the fabrication of the 1DPCs with a few to a dozen layers is a challenge and make additional fabrication cost, the advantage of this light trapping structure is considerable since the 1DPCs can increase the efficiency of the ST-SCs by reflecting all the photons with energies less than the photon bandgaps (PBGs) for re-absorption in the active layer.
It should be mentioned that the active layer of the ST-SCs cannot completely absorb the reflected light by the 1DPCs since the thickness of the active layer is so thin. For conventional opaque OSCs, some methods have been previously developed and applied for light trapping. The methods are based on optical spacers, surface plasmons, and optical microcavity [23][24][25][26][27][28][29][30] . Using optical microcavity in combination with optical spacers can confine a large number of photons within the device and increase light absorption in the active layer 25,30 . To increase the absorption of reflected photons from 1DPC in the active layer of ST-SCs, considering the improvement in absorption is usually accompanied by a reduction in transparency, it is even more challenging to develop light-trapping structures which can improve photon absorption without reducing the transparency of semitransparent devices. www.nature.com/scientificreports/ This paper aims to develop a light-trapping structure to enhance photon absorption in ST-SCs using a semitransparent microcavity. For this purpose, the active layer is sandwiched between the MoO 3 /Ag/MoO 3 multilayer electrode and the Ag electrode which is capped by 1DPCs. The proposed structure without 1DPC has been previously studied by our group in terms of different structural parameters to improve efficiency and transparency, and an efficiency of about 4% with 45% transparency is reported. In this paper, the theoretical calculations based on Transfer-Matrix Method (TMM) 31,32 show that the semitransparent microcavity can improve the power conversion efficiency of ST-SCs. The effects of the variable pairs of 1DPCs on the power conversion efficiency of ST-SCs and their transparency are also investigated.
Another commonly reported figure of merit for ST-SCs is light utilization efficiency (LUE). Taking into account both PCE and AVT, a direct comparison of LUE values can hold viable information in contrast to the direct comparison of the AVT values without knowledge of the PCE 33 .
Achieving widespread adoption of semi-transparent organic solar cell technology requires combined optimization of PCE and AVT. While electronic displays require AVT > 80% (LUE > 5%), architectural tinted glass requirements typically start closer to 50%. PCE values of 5-10% (LUE > 2.5%) are required in BIPV (Building Integrated PV) applications to reduce electricity costs. However, 2-5% PCE (LUE > 1.5%) is sufficient for low-power mobile electronic devices. TPVs with similar PCE but lower AVT (LUE > 1%) can self-power smart windows or complement passive window coatings 34,35 . Recently, a high-order LUE transparent solar cell (LUE = 5.46%) with an efficiency of 9.1% and an AVT of 60% has been reported. Another reported work with LUE = 2.2% concerns a wavelength-selective DSSC device (PCE = 6.1%, AVT = 36%) with a low-cost diphenylamine-based dye and a highly transparent iodine-free electrolyte. Among inorganic translucent devices, CIGS cells are the ones with the best performance of LUE = 1.3%. Other inorganic semi-transparent devices have been demonstrated with Cu 2 ZnSn(S, Se) 4 and Sb 2 S 3 -based solar cells with still low performance (LUE < 1%) 36 .

Device structure and theoretical model. The modeled ST-SCs consist of Glass/MoO 3 (I)/Ag/MoO 3 (II)/
Active layer (PBDB-T:ITIC)/TiO 2 /Ag/LiF/1DPCs structures. Where MoO 3 /Ag/MoO 3 acts as the transparent top electrode, and the inner MoO 3 layer (30 nm) acts as the HTL. Sol-gel processed ZnO layer is used as the ETL, and ITO is the transparent bottom electrode. MoO 3 (I) /Ag also acts as the input antireflection layer with a thickness assumed to be 10/6 nm, and Ag/LiF/1DPCs act as the output antireflection layer with thickness set as 10 nm, and different numbers of pairs of 1DPC to construct semitransparent microcavity and then improve the optical performance of the ST-SCs.
The thickness of the electron transport layer (TiO 2 ) is set to 10 nm and the thickness of the active layer (PBDB-T: ITIC) is assumed to be 100 nm. The phase matching layer (PML), LiF film is sandwiched between the Ag electrodes and set to 84 nm. The 1DPCs are assumed to be composed of different pairs of WO 3 /LiF, as is illuminated in Fig. 1. The layer thicknesses of the WO 3 and the LiF are determined by Eq. 1 37 : where n WO3 and n LiF denote the refractive index of the WO 3 and LiF layers, and d WO3 and d LiF denote the thickness of WO 3 and LiF layers, respectively; λ is the center wavelength of the photonic bandgap of the 1DPCs. A schematic view of the structure of glass/MoO 3 (I)/Ag/MoO 3 (II)/active layer/TiO 2 /Ag/PML/1DPCs layers which according to thin film optics 30 acts as a microcavity is shown in Fig. 1.
The transmission of the input (MoO3(I)/Ag/MoO3(II)) and output (TiO2/Ag/PML/1DPCs) mirrors of the constructed microcavity for the passband wavelength of the 1DPCs (300 nm-1000 nm) is shown in Fig. 2. As shown in the figure, the related passband confirms the device transparency after using the microcavity. Knowing www.nature.com/scientificreports/ the reflectance (R1, and R2) and transmission (T1 and T2) of the cavity mirrors, the transmission of the device can be calculated using thin film optics and is expressed as 38 : where β = 2πkd/ and k denote the extinction coefficient of the active layer.
To model the device transparency, the transmittance of all individual layers, and all together, are calculated using the TMM method 25 . It should be reminded that the transparency properties of a device are determined by both average visible transmittance, AVT, and by transmittance characteristics in the visible light wavelength range (370-740 nm), taking into account the photopic response of the human eye V(λ).
To calculate the device's performance parameters such as short-circuit current (Jsc), open-circuit voltage (V oc ), fill-factor (FF), and power conversion efficiency (PCE), we used the drift-diffusion model, where, in addition to considering the density of excitons, their displacement is taken into account. The calculated AVT value is extensively explained in our previous publications 39 . We have used Eq. (3) to consider the effects of PCE and AVT simultaneously (LUE) 33 : To qualify the implementation in practical applications such as architectural window glass and mobile surfaces, aesthetics are just as significant as PCE for TPV devices. Aesthetic quality can be quantitatively estimated from three main figures of merit: the AVT, color rendering index (CRI), and the CIELAB color coordinates (a*, b*). The calculation of AVT, CRI, and color coordinates requires the transmittance spectrum of the OTPV as the input data. The CIELAB is a device-independent, 3-dimensional color space that enables precise measurement and comparison of all perceivable colors using three color values.
In this color space, numerical differences between values correspond to the amount of change humans see between colors, which was defined by the International Commission on Illumination (abbreviated CIE) in 1976.
It expresses color as three values: L* for perceptual lightness and a* and b* for the four unique colors of human vision: magenta, green, blue, and yellow. In this paper, we have reported the CIELAB color space parameter set (a*, b*), which indicates the relative color concerning a reference illumination source 34 .
With, www.nature.com/scientificreports/ where X , Y,Z are the tristimulus values of the test object color stimulus considered and X n , Y n , Z n are the tristimulus values of a specified white object color tristimulus. Generally, the specified white object color stimulus should be light reflected from a perfect reflecting diffuser illuminated by the same light source as the test object 36 .

Result and discussion
To calculate the performance parameters of the ST-SCs, two different structures (Device A and B) have been taken into account and the obtained results are compared and the key parameters are shown in Table 1. Device A (a ST-SC with a microcavity) has a structure of Glass/MoO 3 (10 nm)/Ag (6) Fig. 3. The amount of transmission in device A is reduced for all wavelengths, but this reduction is less for the eye sensitive area (orange color in Fig. 3). Reduction of the transmission spectrum means that the absorption spectrum for Device A is improved and this improvement is attributed to the resonance effects of the semitransparent microcavity.
In the next step, we investigated the effects of the number of photonic crystal layers on cells' efficiency and transparency. We have calculated the transmission of the devices with the structure including 4 to 20 pairs layers of photonic crystal and related results are shown in Fig. 4. The results show that the number of photonic crystal layers does not have a great impact on the overall transparency and therefore on the performance parameters of the solar cell, although the behavior of transmission for different wavelengths is slightly different from a different number of photonics crystal layers. The effects of these pairs of layers on the PCE, AVT, and LUE of ST-SCs are investigated and depicted in Fig. 5, and Table 2.
Calculations show that the photonic crystal with 8 pairs of layers in the structure can change the value of PCE and AVT, so It's found that PCE improved from 7.52 to 8.72% and AVT decreased from 25.9.19 to 22.24%. By increasing the number of layers, there is not much change in PCE and AVT. Therefore, PCE can be increased with microcavity even with a minimum number of pairs of layers (at least two pairs). Also, from Fig. 5 and Table 2, it could be found that there isn't a significant difference between the reported values of LUE for different pairs of 1DPCs, however, there is an optimum situation in the case of 8-12 pairs of 1DPCs.
The CIE color space, including the coordinates of ST-OSC for both devices (with and without microcavity), is shown in Fig. 6. The color coordinates of both devices are very close to each other and located beside the color point or so-called "white dot" in the CIE chromaticity diagram. Using microcavity changes the color coordinates slightly, but the device can transmit high-quality light with a near-white sensation to the human eye with a very small changing the original color of an object.   www.nature.com/scientificreports/ It should be pointed out that the PCE and AVT data in this paper are calculated using the model reported in our previous article 37 . As usual, the practical device PCE and AVT may be smaller than the calculated PCE and AVT data proposed in this paper. But, optical modeling in this paper can provide in-depth knowledge on how to apply semitransparent microcavity to simultaneously improve the photon absorption and transparency of the semitransparent OSCs. Predictive and descriptive research results obtained from these researches are considered very helpful for the design, fabrication, and experimental study of 1DPCs-based semitransparent OSC devices. Figure 7 represents the CIE LAB color space parameter set (a*, b*) of the ST-OSCs in two cases: with and without using microcavity. As expected, similar to the CIE color coordinate space diagram, for both cases, the values of a* and b* are nearby to each other located close to the white point.  www.nature.com/scientificreports/ In Fig. 8a, we have compared the LUE versus AVT for our cases, without and with 8-pairs of 1DPCs, with some reported experimental data. As shown in the figure, the LUE does not change, and there is a very small change in AVT. Also, the calculated LUE for the optimum structure (8-pairs layers of 1D-PC), is in acceptable coincidence with the similar ST-OPVs. Figure 8b, shows the PCE versus AVT for our cases, without and with 8-pairs of 1DPCs, in comparison with some reported experimental data. As depicted in the figure, the PCE is changed 36 .

Conclusion
In this paper, the light trapping structure based on semitransparent microcavity is constructed for ST-SCs by sandwiching an active layer between the (MoO 3 /Ag/ MoO 3 ) multilayer electrode and a thin Ag electrode capped by 1DPCs. We have investigated the effects of different pairs of 1-dimensional photonic crystals (1DPCs) on the ST-SC's properties such as the PCE, AVT, LUE, the color coordinates in the CIE color space, and CIE LAB. As a result, the presence of microcavity can improve the power conversion efficiency by about %16 in comparison with the absence of microcavity. Although the transmission is decreasing slightly (14% decreasing), using microcavity does not change the color coordinates much and the device can transmit high-quality light with near-white sensation to the human eye.

Data availability
The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.